A time delay and integration (TDI) function operates on the premise that two or more radiation detector channels, which are disposed colinearly in a scan direction and which view the same part of a scene during a given scan, will exhibit relative signal levels that are correlated, although separated in time. However, the noise associated with each channel is not correlated. By temporal phasing (a time delay function) and adding the radiation detector channel signals (an integration function) the resultant output signal has been found to exhibit a signal-to-noise ratio that is superior to that of a single detector channel. This is because the signals sum, but the noise is root sum squared, resulting in a signal-to-noise ratio improvement equal to the square root of the number of detectors.
By example, it is assumed that an object, sensed at each detector as a spot with relative brightness, moves from a detector A to a detector B. The spot covers detector A at some time T1 and a sample at detector A is taken at that time. At some later time, T1+t, the spot covers detector B and another sample is taken at detector B. By adding the sample from detector A that was obtained at time T1 to the sample from detector B that was obtained at time T1+t, a higher signal-to-noise ratio estimate of the intensity of the spot is obtained.
Many conventional imaging systems implement the TDI function by causing all of the detectors to simultaneously take a sample of what the detectors are “seeing”. This is often accomplished by sampling the detector circuitry through a switch that is coupled to a hold capacitor. When the switch is closed the voltage of the signal is placed on the capacitor. When the switch is opened this voltage remains on the capacitor until the switch is once more closed.
Conventional approaches for implementing the TDI function typically employ charge coupled devices (CCDs) or bucket-brigade device (BBD) circuits. As the name suggests, CCDs operate on a charge mode, and operate by propagating and summing charges. However, CCD circuits generally require non-standard and/or additional semiconductor processing than does conventional metal-oxide semiconductor (MOS) device fabrication, raising costs. Additionally, certain applications for arrayed detectors or sensors, such as LandSat and Ikonos remote satellite imaging systems, require radiation tolerant hardware. One figure of merit reflecting radiation ‘hardness’ of electronic devices is total ionizing dose, and CCDs typically exhibit a low tolerance for total ionizing radiation. This entails further specialized processing in fabricating systems using a CCD TDI arrangement. In addition, both CCDs and bucket brigades suffer from transfer inefficiencies when the number of detector channels in TDI, that is samples per pitch, becomes large.
Commonly assigned U.S. Pat. No. 4,970,567, entitled “Method and Apparatus for Detecting Infrared Radiation”, issued Nov. 13, 1990 (W. L. Ahlgren et al.) discloses a monolithic structure for detecting radiation having a substrate having read out signal processing electronics integrated thereon. Commonly assigned U.S. Pat. No. 5,149,954 describe a MOS fabricated TDI circuit for each pair of detectors that equilibrates alternating capacitors with a common capacitor at each time frame.
What is needed in the art is a robust and inexpensive circuit and method for integrating signals from a line of sensors, especially a multi-dimensional array of sensors. In short, what is needed is a circuit and method offering the low light and resolution advantages of CCDs, without their current expense.